Previously synthesized D5-phenylalanine, D4 tyrosine and D5 tryptophan will be incorporated into L. casei enzymes for NMR spectroscopy. D4 (aminobenzoyl) folic acid and aminopterin will be prepared for these studies. 13C labeled histidine and tryptophan will be prepared for similar studies by CMR. Folic acid and aminopterin labeled with 13C at the 2 and 4 positions and in the benzoyl carbonyl will be prepared for CMR studies of folate enzymes. Affinity matrices for dihydrofolate reductase and thymidylate synthetase will be supplied for purification of these two enzymes for both NMR and X-ray crystallography.